The invention relates to a method for determining the air mass which is supplied to an internal combustion engine via an intake manifold. An underpressure is applied to an underpressure store of a servo system via the intake manifold. The air mass flow is determined in the inlet region of the intake manifold.
A method of the kind referred to initially herein is conventional in motor vehicles in the marketplace. A sensor is mounted in the region of the intake manifold of the engine and is usually a hot-film sensor which measures the air mass flow arriving in the intake manifold from the inlet thereof. This sensor signal is transmitted to a central electronic control unit which uses the signal to adjust the injection quantity of the fuel (if required, the injection time point of the fuel as well as the ignition time point) in dependence upon the desired torque and in view of the lowest possible fuel consumption and the lowest possible toxic substance emission.
However, in motor vehicles, evermore servo systems are used which assist the user. These systems include, for example, the following: brake booster, power steering as well as the control of air flaps of a climate control system.
However, it has been shown that there are deviations of the toxic substance emissions from the optimal values when actuating specific servo systems.
It is an object of the invention to provide a method which will avoid the above-mentioned deviations.
In a combination of an internal combustion engine and a servo system having an underpressure store, the method of the invention is for determining the air mass supplied to the engine via an intake manifold thereof. The method includes the steps of: applying an underpressure to the underpressure store via the intake manifold; determining the air mass flow in the inlet region of the intake manifold; detecting an actuation of the servo system; and, correcting the determined air mass flow at or directly after the detected actuation of the servo system.
The measures according to the invention are based on the following considerations. The energy for operating pneumatic servo systems is made available by an underpressure store which is fluidly connected to the intake manifold. The underpressure, which is necessary in the store for operating the servo systems, compared to the ambient pressure is effected by evacuating the underpressure store based on the underpressure present in the intake manifold.
The detection of the air mass flow must take place in the inlet region of the intake manifold because of flow and geometric reasons. However, the maximum underpressure in the intake manifold is achieved only downstream of this location. For these reasons, and when evacuating the underpressure store, air from the underpressure store flows into the intake manifold and is not detected by the sensor measuring the air mass flow. This, in turn, has the consequence that more air reaches the combustion chamber of the engine than was detected by the sensor. This finally leads to the situation that the mixture is leaner than assumed by the electronic control unit and this negatively influences the emission of toxic substances.
According to the invention, it was recognized that air masses from the underpressure store only flow into the intake manifold when a servo system, which is connected to the underpressure store, is actuated. Accordingly, and in accordance with the invention, a corrective term is added to the determined air mass flow when a servo system is actuated and the conditions, which form the basis of the computation in the electronic control unit, approximate the real conditions so that the toxic substance emissions are affected only to a slight extent or not at all by the actuation of the servo system.
Basically, it would be possible to add a constant corrective term to the measured air mass flow when a servo system is actuated. Actually, the air mass flow, which flows from the underpressure store into the intake manifold, is, however, dependent upon the pressure difference between the pressure present in the underpressure store and the pressure present in the inlet region of the intake manifold. According to another embodiment of the method of the invention, this is accounted for with the following steps:
(a) determining the pressure present in the intake manifold;
(b) determining the pressure present in the underpressure store;
(c) computing the additional air mass which flows from the pressure store into the intake manifold because of an actuation of the servo system; and,
(d) correcting the air mass flow when an actuation of the servo system is detected, if required, by adding the additional air mass to the mass flow determined in the intake manifold.
For reasons of cost and for geometric reasons, it is advantageous when a pressure sensor in the intake manifold can be omitted. This is made possible in accordance with a further embodiment of the method of the invention whereby the pressure in the intake manifold is computed from the corrected mass flow based on an intake manifold model.
Costs can be further saved in that no pressure sensor is used in the underpressure store. Here, a method is appropriate with which the pressure, which is present in the underpressure store, is determined in that the mass flow supplied to the underpressure store and drawn out of the underpressure store is carried out.
In another embodiment of the method of the invention, the intensity of the actuation of the servo system is detected and the correction of the air mass flow is correspondingly adapted. This makes possible an especially exact correction of the air mass flow and a further reduction of the emissions.
Alternatively or in addition thereto, the correction can be so carried out in accordance with another embodiment of the invention that essentially no deviation of the air/fuel ratio (lambda value) from a desired value takes place when an actuation of the servo system is detected. The correction of the air mass flow here takes place therefore in dependence upon the lambda value, which is determined in the exhaust-gas pipe, and is therefore especially exact. This air mass flow forms the basis for the computation of the fuel quantity to be injected, the ignition time point, et cetera.
Another embodiment of the method of the invention is especially useful wherein the intensity of the correction of the air mass flow is used to monitor the operation of the underpressure store when an actuation of the servo system is determined. When the corrective term (required for the adjustment of the desired lambda value) for the air mass flow exceeds a pregiven limit value (that is, an air mass, which lies above a limit value, is evacuated from the underpressure store into the intake manifold), then this permits a conclusion to be drawn as to a leak in the underpressure store or in the brake booster. In such a case, a bit can be set, for example, in the electronic control unit, which permits a warning display to illuminate in the dashboard of a corresponding motor vehicle.
Another embodiment of the method of the invention goes in the same direction wherein the size of the deviation of the air/fuel ratio (lambda value) from a desired value is used for monitoring the operation of the underpressure store when an actuation of the servo system is determined.
The invention also relates to a computer program which is suitable for carrying out the above-described method when it is executed on a computer. It is especially advantageous when the computer program is stored on a memory, especially on a flash memory.
The invention relates finally also to a control system for an internal combustion engine, especially of a motor vehicle. An air mass is supplied to the engine via an intake manifold. An underpressure is applied via the intake manifold to an underpressure store of a servo system and the air mass flow in the inlet region of the intake manifold is determined.
According to the invention, it is provided that the control system detects the actuation of the servo system and corrects the determined air mass flow when an actuation of the servo system is detected.